24 THE FUNCTIONS OF ANIMALS. 



others depend for their carbon supplies on the sugar, starch, 

 and fat already made by other animals, or by plants. As 

 regards nitrogen, most plants take this from nitrates and 

 the like, absorbed along with water by the roots ; whereas 

 animals obtain their nitrogenous supplies from the complex 

 proteids formed within other organisms. Most plants, 

 therefore, feed at a lower chemical level than do animals, 

 and it is characteristic of them that, in the reduction pf 

 carbon dioxide, and in the manufacture of starch and 

 proteids, the kinetic energy of sunlight is transformed by 

 the living matter into the potential chemical energy of 

 complex foodstuffs. Animals, on the other hand, get their 

 food ready made \ they take the pounds which plants have, 

 as it were, accumulated in pence, and they spend them. 

 For it is characteristic of animals that they convert the 

 potential chemical energy of foodstuffs into the kinetic 

 energy of locomotion and other activities. In short, the 

 great distinction an average one at best is that most 

 animals are more active than most plants. 



Chief functions of the animal body. There are two 

 master activities in animals, those of muscular and of 

 nervous structures; the other vital processes, always ex- 

 cepting growth and reproduction, are subservient to these. 

 Let us now consider these master and subsidiary functions, 

 as they occur in some higher organism, such as man. 



Nervous activities. Life has been described as action 

 and reaction between the organism and its environment, 

 and it is evident that an animal must in some way become 

 aware of surrounding influences. 



An external influence stimulates a sensory cell or its 

 ending, and a message travels by a sensory fibre to the 

 nerve-cord. The inner end of the sensory fibre is con- 

 nected with the branches or dendrites of an associative 

 (communicating or internuncial) cell. Thence the message 

 is passed, still within the cord, to the dendrites of a motor 

 nerve-cell. Thence an efferent impulse travels down the 

 axis-cylinder or motor nerve-fibre of the motor neurone to 

 an ending on a muscle-fibre, which is thus commanded to 

 contract. The whole nervous system is essentially a con- 

 nected series of such reflex-arcs, all intricately joined up 

 with one another. 



